In the manufacture of paper, board and similar cellulosic products, a headbox directs a dilute stock slurry consisting of about 99% water together with about 1% papermaking fibers and other components onto a forming fabric in the forming section of a papermaking machine. The forming fabric serves to provide uniform support for the papermaking fibers while allowing a sufficient portion of the water to drain through so that, upon reaching the end of the forming section, a very wet but cohesive embryonic web of fibers remains. This web is then transferred to the press section of the machine where a further proportion of water is removed by mechanical pressure. As the web is transported through the press section upon or between one or more press felts, it passes, together with the fabrics, through at least one press nip formed between two opposed rolls. Following the press section, the web is then passed to the dryer section where it is conveyed over a series of heated rolls so that the remainder of the water is removed by evaporation.
The press felts play an important role in the papermaking process: they provide a reservoir into which water that is expressed from the web can be carried for subsequent removal, and they provide a uniform, non-marking surface upon which the web is conveyed as it passes through the press nips. Press felts are typically constructed using a woven base fabric into which is needled a number of layers of a relatively fine fibrous batt material; typically, both the base fabric yarns and the batt fibers are comprised of nylon or similar polymer.
Until recently, press felts were made in one of two ways. In the first method, the base fabric was woven endlessly, similarly to a sock or tube so that it did not include a seam, and according to the size required to suit the press section into which it was to be installed. Once this base fabric was completed, layers of batt material were then needled into it and the press felt was then finished according to known methods. When installed, it was slipped over the press section area for which it was made. In the second method, the base fabric was woven flat to the width and length of the final felt, a seam area created at the opposing longitudinal ends, and layers of batt needled onto the base fabric. The base fabric was typically a multilayer weave construction to provide dimensional stability and void volume to the final press felt; the seam area was constructed to allow for joining the fabric on the machine for which it was intended by passing it through the press section while attached to the older fabric it was to replace. Both methods and constructions have been in use for a number of years.
It has more recently been proposed to manufacture press felts using relatively narrow (about 1 m) wide strips of woven base fabric, rather than the prior art base fabrics described above which are woven to the intended full width of the final product. See, for example, U.S. Pat. No. 5,268,076 to Best et al. and U.S. Pat. No. 5,360,656 to Svensson et al. As disclosed in those patents, a continuous narrow woven base fabric strip is spirally wound to provide the desired length and width of the final fabric. This manufacturing method has met with great success because it is now no longer necessary to use very wide industrial looms to manufacture the base fabric as was done previously. Instead, a relatively narrow loom can be used to produce a continuous strip of woven base fabric material which is then assembled according to the spiral winding process in which the longitudinal edges of adjacent turns of the helically wound strips are bonded one to the next by one of various methods.
In the spiraling process, a fabric strip is led from a relatively narrow high speed loom and directed around at least two large diameter rolls set a desired distance apart such that it is canted by a small angle of from about 1° to about 10° to the intended machine direction of the final product. Each successive turn of the fabric strip is laid adjacent to, and bonded to, that previously supplied so as to build up sufficient length and width of a cohesive, tube-like precursor base fabric with no widthwise seam. This precursor base fabric is then removed from the spiral winding apparatus and collapsed onto itself so that fold areas can be formed at the opposed widthwise ends. The fold areas will be used to install a seam at a later point in processing; the collapsed tube, from fold to fold, is now approximately equal to the required length of the final fabric.
When the precursor fabric is flattened in this manner, the spirally wound woven material from each side of the tube is brought into intimate contact with one another. Because the entire base fabric is woven according to the same weave pattern, interference patterns appear at locations where the warp and weft yarns from the opposing surfaces overlap but are not in exact alignment, forming distributed regions of low and high yarn densities throughout the fabric. A press felt made using this base fabric will also have areas of low and high yarn density which, when in use, will cause uneven water removal from the web and thus sheet marking. In addition, adhesion of the batt fibers to the base fabric will be irregular rather than uniform throughout because these fibers will find “better” anchorage in regions of the base fabric where yarn densities are comparatively higher. The press felt will tend to shed batt fibers more quickly during use from those areas where base fabric yarn density is comparatively lower. This in turn will affect the uniformity of the fabric and thus its overall ability to dewater the sheet evenly without imparting a mark to it.
This interference phenomenon in multiaxial press felts (also known as the Moire effect) is well known. WO 2004/099496 to Herring et al. teaches that interference patterns can be eliminated by introducing a degree of randomness into the weave of the fabric by arranging at least a portion of the weft yarns so that they do not follow a linear path. U.S. Pat. No. 7,473,336 to Hawes et al. discloses a base fabric in which machine direction (MD) and cross-machine direction (CD) yarns which are arranged in a predetermined manner such that a distance between one pair of adjacent CD yarns is different than that between another pair of adjacent CD yarns.
While both of these solutions to this problem have met with limited success, neither provides for a press felt base fabric made from a single layer woven structure. Further, it is difficult to employ either technique in a manufacturing environment as both require manipulation of loom settings during production, which is undesirable. Single layer base fabrics are desirable because they will provide for a comparatively lighter press felt than will a similar base fabric made using a double layer weave design, and will possess unique compressive properties which makes them suitable for certain papermaking applications which have relatively low nip intensities and/or are required to handle less water.
Similar problems occur in full width, flat woven press felt base fabrics, which can be produced as two separate full size pieces which are then “socked” one inside the other and then needled together. This construction can also result in interference patterns occurring between the two layers, forming regions of relatively low and high yarn densities as a result of the misalignment of the yarns in the two opposing surfaces, and thus uneven water removal and sheet marking for the reasons previously described.